This invention relates to internal combustion engines and, more specifically, to speed controlled hydraulic valve lifters in such engines. The invention is particularly advantageous when used in reciprocating diesel engines but may also be employed with efficacy in reciprocating spark ignition engines.
Reciprocating diesel engines of the type installed in vehicles must be capable of starting at low ambient temperatures and quickly warming up with a minimum of generation of so-called "white smoke" an undesirable emission, which is typical of the operation of a diesel engine which is cold. Moreover, since such engines are frequently driven by the vehicle, as when the vehicle is coasting downhill, valve train separation must not be accompanied by a significant growth in the length of the train since such growth may cause interference between the piston and the valve heads. Such interference inevitably bends the valves, or, in severe cases, breaks the valve heads from the stems causing extensive damage to the engine.
For the foregoing reasons, hydraulic valve lifters have not been used to any appreciable extent in diesels used for driving vehicles. The undesirable pumpup characteristics of hydraulic valve lifters cause the aforementioned undesirable growth in the length of the valve train at high speeds.
In a typical vehicular diesel, there is also considerable overlap in the operation of the valves. Typically, the intake valve will close after bottom dead center to make use of the inertia of the air within the manifold in the port to provide a ram effect which results in trapping more air within the cylinder. The exhaust valve is opened considerably before bottom dead center is attained in order to provide sufficient time for exhaust blowdown to take place and minimize back pressure. At the end of the exhaust process, the intake valve has already opened a considerable amount which produces mixing of the fresh air charge with combustion products at low engine speeds but not at higher speeds. Such valve overlap is harmful at low speeds but is beneficial at higher speeds because of improved breathing.
Moreover, because the intake valve closes after bottom dead center, at low speeds or during cranking for startup, a certain amount of the air in the cylinder in expelled backwardly into the intake manifold to lower the design effective compression ratio, for example, from 18:1 down to 17:1. As a consequence, the gas temperature at top dead center during the compression stroke, where fuel is injected, is lowered thereby making the engine more difficult to start and/or decreasing operating efficiency at low speeds.